The Gemini Planet Imager is an extreme adaptive optics system that will employ an apodized-pupil coronagraph to make direct detections of faint companions of nearby stars to a contrast level of the 10-7 within a few lambda/D of the parent star. Such high contrasts from the ground require exquisite wavefront sensing and control both for the AO system as well as for the coronagraph. Un-sensed non-common path phase and amplitude errors after the wavefront sensor dichroic but before the coronagraph would lead to speckles which would ultimately limit the contrast. The calibration wavefront system for GPI will measure the complex wavefront at the system pupil before the apodizer and provide slow phase corrections to the AO system to mitigate errors that would cause a loss in contrast. The calibration wavefront sensor instrument for GPI has been built. We will describe the instrument and its performance.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.